Silver nanowires on acid-alkali-treated titanium surface: Bacterial attachment and osteogenic activity

Abstract

In the present work, a nanoscale porous surface was gradually prepared on Ti surface by two-step method of acid corrosion and NaOH–H₂O₂ mixing treatment. This led to the formation of a TiO2 coating at the surface of the underlying Ti substrate. Then, silver nanowires were deposited on the nanostructured surface by spin-coating technology. The nanowires were found to be evenly introduced into the nanostructured TiO2 coating. The modified Ti surface showed good wettability and protein adsorption capacity. More importantly, in-vitro cellular and bacterial assays indicated that the modified Ti surface possessed not only the excellent adhesion and differentiation abilities for osteoblasts but also relatively strong antimicrobial capacities for both S. aureus and E. coli. The results suggest that such modified Ti surfaces greatly prefer the initial adhesion of osteoblast-like cells over the initial adhesion of bacteria. The surface modification that we made to Ti substrate can promisingly meet clinical requirements.

Introduction

Used as plastic surgery and dental implants, porous titanium-based materials can reduce stress-shielding effect [1,2], increase contact bone area, accelerate bone ingrowth, and lead to strong biomechanics interlocking [[1], [2], [3], [4]]. However, their lack of antibacterial ability makes titanium-based biomaterials more prone to postoperative implant failures because of implant-related infections [5,6]. The surface of a titanium-based implant may host colonies of bacteria which form biofilms as a way of protection against external stimuli, which leads to different chronic infections [7]. The use of organic and inorganic antimicrobials can enhance the surface antibacterial ability of Ti-based biomaterials [8,9].

Antibiotics can be in-situ loaded onto the surface of titanium-based implants by the “nano-reservoir” effect [10] or chemical bonding [11], which is however of considerable concern due to antibiotic resistance [12,13]. Therefore, nano-antibiotics, nanomaterial-based antimicrobials like silver nanoparticles, are promising candidates against implant-related infections [12,14,15], though any possible cytotoxicity of free ions or nanoparticles should also be taken into consideration. Moreover, a desired implant surface should keep a balance between antibacterial and cellular functions [16,17]—the surface should not harm the normal functioning of cells, while killing bacteria. This can be promisingly achieved by nanotechnology [18,19].

Silver-incorporated titanium implants can keep a balance between antibacterial and cellular functions, though silver ions released from implants make silver nanoparticles toxic to surrounding tissue [20,21]; therefore, biocompatibility of titanium-based implants is the third important factor that needs to be seriously taken care of. Silver nanowires (AgNW) have been shown to lower the release of silver ions than what silver nanoparticles do, while still effectively enhancing antibacterial functioning of titanium surfaces [22]; this introduction of nanowires of silver as antibacterial agents into the field of bio-coating is expected to open a whole new avenue of research into the fighting against a series of pathogens on biomaterials surfaces. Another factor of high importance in synthesizing an implant is its ability of stimulating bone tissue regeneration; a hierarchically nano-structured titanium surface well resembles mixed structure of human bone tissue and extracellular matrix, and can therefore better enhance the functioning of cells [23,24].

A roughened surface on a porous titanium substrate can be prepared with chemical modification such as acid, alkali, and acid-alkali treatments [6,[25], [26], [27], [28]]. Such treatments do not destroy surface irregularities which are helpful for bone ingrowth [6,25]. Pits or grooves produced through acid treatment produces on a Ti substrate are of micron size, and those left by alkali treatment are of nano scale [3,26]; that is, a combination acid-alkali treatment can potentially lead to micor- and nano-scale network structures on the surface, which enhances osteoinduction [1,4]. Therefore, we can already conjecture that we may arrive at a well-armed Ti-based implant surface which desirably enjoys both antibacterial and cellular functions if we combine the benefits of silver nanowires and a highly-porous micro-/nano-structured surface.

The realization of the above-mentioned idea is what we are after in the present work. We fabricated a highly-porous micro-nano-structured Ti substrate by a two-step method of acid corrosion and NaOH–H₂O₂ mixing treatment. Thereafter, the stabilization of the silver nanowires on the micronanostructured substrate was achived by the spin-coating technology. Then, the so-fabricated Ti surface was studied for its antimicrobial ability, and its initial adhesion, differentiation, and proliferation of human osteosarcoma MG63 cells.